Radio receiving system



1942. R. c. DICKINSON 2,300,375

RADIO RECEIVING SYSTEM Original Filed April 19, 1941 Oscillafor FirstDetector LFAmplifier Amplifier; I

WITN ESSES. INVENTOR 5058!? C Die/hm on.

ATTORNEY Patented Nov. 3., 1942 Q l UNITED STATES PAT-'ENT 'QFFI aszi itizzsrs'mm i 7 Q to Westinghouse Electric & Manufacturing. Company,Pittsburgh,-:Pa., a corporation l 1 of Pennsylvania Original applicationApril 19, 1941,'Serial -No. 389,353. Divided and this applicationJanuary 28, 1942, Serial No.428,48'8- 1 Claim.

This invention relates to radioreceivin'g sys temsand more particularlyto circuits for eliminating interference caused by static and similartransient surges affecting. the receiver.

This application is" a" division of my"application Serial No. 389,353,filed April .19, 1941; for Radio receiving systems and assigned to theWestinghouse Electric & Manufacturing Company, which claims certain:subject matter herein disclosed.

The problem of interference due to certain high frequency discharges isan acute. one and various methodshave beenproposed for its eliminationor at. least for lessening the undesirable effect produced thereby. The:nature of the electrical disturbance varies with the source of theinterfering. radiation and those originating in the electrically chargedatmosphere'and reaching the receiver in the form of highly dampedelectromagnetic impulses are especially diflicult to eliminate.Transients of this type are usually several times greater in intensitythan themcoming carrier frequency energy and affect the demodulator ofthe receiver due to their highly damped characteristics by impactexcitation.

The primary object of this invention is to cancel the effect oftransient impulses reaching the demodulation circuit of the receiverbyproviding means for automatically controlling the output impedance ofthe demodulator tube upon sudden rise of input voltage and for aduration of time necessary for the decay of transient oscillatorycurrents.

Another object of this invention is to attenuate automatically the audiofrequency energy derived from the demodulator of the receiving systemWhenever the rectified component of the carrier wave exceeds apredetermined level at a transient impulse rate.

A further object ofthis invention is to provide a control circuitconnected to the demodulator portion of the receiver which produces arectifier signal voltage whereby the effective impedance and hence thevoltage output of the demodulator is decreased and'the provision ofmeans associated with the control circuit for of a radio receivingsystem, together with the control circuit in accordance with thisinvention.

Referring to the figure, thevarious portions of the receiving system:herein shown as being of the superheterodyne type are indicated byconventional squares and only the modulator portion is shown in detail,to which the present invention is directed; The receiver may be of anyconventional form inasmuch as this invention is applicable to thedemodulatorqportion of any receiving system as long: as it operates as alinear rectifier of the carrier frequency'energy producing a rectifiedsignal voltage.

The system herein shown :employs the superheterodyne principle wherein.the received carrier frequencyenergy is converted to an intermediatefrequency, the latter being fedtothe demodulator tube l, which, actingas a half- ,applied to the primary winding 4 thereof. The

anode 5 ofthe demodulator tube I'is connected .to the high potentialterminal of the-secondary winding 2 and the cathode 6 of the tube l -isgrounded. Between ground and the other terminal of the secondary winding2 is-Rplaced the initiating its action upon signal voltages exloadresistance IL The intermediate frequency component of the signalvoltageis by-passed by a condenser efiectively'in parallel'with theresistance 1. The audio frequency component of the rectified signallingenergy is taken 01f from a potentiometer which is the load resistance lof the diode l by a sliding contactthereof which connects to the inputcircuit of the audio frequency amplifier. In parallel with thedemodulator tube l'is connected the control tube 9 in such manner thatthe: cathode l0 thereof connects with theanodefi ofthe tube and theanode ll of the tube -9 directly to the; grid of tube I and also to thecathode of. tube I- through aresistor 35 and the common groundconnection.

The control element or grid I2 of the tube 9 is connected in series withresistor l3 to the slidingarm ll of a potentiometer 15. The grid I2 isalso by-passed to ground by condenser Hi.

The potentiometer l5 forms the load resistance of another rectifiershown. here in the form of a dual purpose tube, one portion of whichcomprises a multi-electrode amplifier, the other being a dual anodediode rectifier. The amplifier portion of the dual purpose tube llincludes the control grid IB which is connected to the input circuit ofthe demodulator portion of the receiver by means of conductor l9,coupling condenser 20, and the input impedance 2| in the form of a radiofrequency choke coil connected between grid [8 and ground. The screenelectrode 22 is indicated to receive proper operating potentialand theanode'electrode 23 is connected to the primary winding 24 of atransformer 25,

which is tuned to the intermediate frequency, of

the signalling energy by condenser 26. The free terminal of the primarywinding 24 is indicated to receive the required operating potential forthe anode 23. The secondarywinding 21 ofthe transformer 25 isconnected'fbetweenthe paralleled rectifier anodes 28 and 28' andone-terminal of the potentiometer 15, the other terminal thereof beinggrounded. Condenser 29 serves to by-pass the intermediate. frequencyvoltage,

to ground, the cathode 30 of the tube ll being grounded through biasresistance 3|, and bypassed by condenser 32 in shunt therewith,

In the operation of the circuit herein .described, assuming that thecondition of 'tube 9 is such which will not allow thermionicconductivity, a steady signal modulated at the rate of audio frequencieswill be rectified by the tube l producing a rectifiedsignalvoltageacross: the load resistance 1 thereof; The audio frequencycomponent of the signal is furtherv amplified by the audio frequencyamplifier in order to-be reproduced inithe speaker 39. s

However; the control tube 9 in a state. of conductivity, will offer a'low impedance path for currents in the output circuit and therebyattenuate to a marked extent the signal level due to the fact that itsspace current path is eifectively in parallel with the output circuit ofthe demodulator including the secondary winding 2 of I. F. transformer 3and the load resistance 1. In order that the control tube 9 shall notimpede normal receiving conditions, the intermediate frequency voltageis utilized asta source of steady biasing potential for the grid I2 ofthe tube 9. This is accomplished by the combined amplifying andrectifying .action of tube [1, theinput circuit thereof being energizedthrough-condenser 20 by the intermediate frequency voltage which,in theoutput circuit of this tube, is impressed upon the secondary winding 21of the transformer 25 in an amplified form, and then rectified by thediode portion of the tube appearing in the form-of a unidirectionalvoltage across the potentiometer I5. By adjusting the sliding arm I I ofthe potentiometer l5, a desired magnitude of biasVoltage can be obtainedfor the grid l2 of the tube 9 in order to counterbalance any tendency ofconductivity which proper polarity of. signal voltage between cathodeIll and anode ll of the demodulator voltage would otherwise'=cause.Since-no current flows through anode l I" to resistor 35, the grid 33 oftube l is at thesame-potential as its cathode 6, and the tube l acts asa simple rectifier. Therefore, if the moving arm I4 is set so that thenegative grid voltage upon the grid lz-of tube 9 is just sufficient tocause anode current cut-off, the output impedance of the demodulatorcircuit will be'free' from attenuation, and the signal will be heardundim'in'ished. If an impulse of stray stron ger'tha'n-the signal is nowreceived, the resulting voltage impulse will tend to increase thevoltage between cathode l and anode II of tube 9; Similarly, thisimpulse will increase the bias voltage produced across the controlrectifier load resistance l5, tending to maintain the grid I2sufficiently negative to counterbalance the tendency of conductivitycreated by the rise of anode potential in tube 9. However, due to thetime delay of the resistance capacity combination inthe grid circuit oftube 9 comprising the resistor l3 and condenser l6,

- this impulse will be discharged to ground through the anode ll beforethe grid l2 becomes sufficiently negative to reestablish the conditionof balance and non-conductivity of the tube 9.

Current flow through tube 9 causes resistor 35 to impress a negativevoltage on grid 33 of tube l which will result in an attenuation of thesignali the output of demodulator tube 1. Furthermore, the anoderesistance of the tube 9 is of low value compared to the resistance 1and the .voltage appearing across the output circuit of'the'demodulatoris momentarily reduced to a very low value. Stable conditions arereached after the passage of the transient impulse and thesignal isagain detected and passed to the audio frequency amplifier in a normalmanner.

Another utilitarian feature of this circuit arrangement may be found inthe elimination of inter-channel noise. The negative voltage of the,grid ,l2 .will vary with the steady signal strength with thecorresponding fluctuation in the positive anode voltage of the tube 9,so that the latter will be maintained slightly above cutoff. It is thus.apparent that with. no signal,the tube 9 acts as a low impedance shuntacross the rectifier I, which substantially silences the receiver whenno signal is present.

I claim as my invention:

In a demodulation system, a high-frequency signal input and a signaloutput circuit, a demodulator tube connected to said output circuithaving anode, cathode and grid electrodes, producing a rectified signalvoltage,- an impedance element between said grid and said cathode, saidgrid being thereby biased with respect to said cathode, means foraltering the condition of said grid whereby the signal output of saiddemodulateris attenuated, comprising a vacuum tube having anode, cathodeand control electrodes connected to said output circuit and energized bysaid rectified signal voltage in proper polarity of thermionicconductivity, said impedance element being in series with thespacecurrent path of said vacuum tube, and said current producing a voltagedrop thereacross, and means for controlling the space current of saidtube comprising a source of unidirectional voltage varying in accordancewith the high-frequency signal voltage in 'said input circuit, a circuitinterconnecting the control electrode of said vacuum tube and saidsource including a time delay network, said circuit being so adjustedthat said unidirectional voltage maintains the grid electrode of saidvacuum tube at a potential of space current cutoff within signal inputlevels of normal magnitude and frequency range, said time delay networkhaving such characteristics as to introduce sufficient lag in theapplication of said unidirectional bias voltage at signal magnitudebeyond modulation voltage intensities as to cause instantaneousconductivity of said vacuum tube attenuating thereby the rectifiedsignal voltage of said demodulator tube.

' ROBERT C. DICKINSON.

